Hydraulic drive



April 14, 1953 T. KLATTE EI'AL 2,634,582

HYDRAULIC DRIVE Filed June 19, 1951 2 SHEETS-SHEET 1 INVENTORS: THEODQR KLATTE' MD 3 "URL/01$ April 14, 1953 T. KLATTE EI'AL HYDRAULIC DRIVE 2 Sl-lEETS-Sl-IEE'I' 2 Filed June 19, 1951 WWE INVENTORSIJ ATTE A 0770 Mam/ra- Patented Apr. 14, 1953 UNITED STATES HYDRAULIC DRIVE Theodor Klatte, Eremen-Huchting, and Otto Niibling, Berlin-Frohnau, Germany; said Niibiing assignor to said Klatte Application June 19, 1951, Serial No. 232,252 In Germany June 28, 1 950 11 Claims.

ineflective if a conduit of the hydraulic drive breaks, or if too much fluid is lost on account of leaking.

It is the object of the present invention to obtain a reliable operation of a hydraulic drive by means of a safety device which stops or brakes the fluid engine when a break or leakage occurs in one of the conduits of the hydraulic drive.

It is another object of the present invention to provide in the return conduit of a hydraulic drive a shut-oil valve controlled and actuated by the variations of the fluid conditions in a reservoir communicating with the conduits of the hydraulic drive.

It is a further object of the present invention to provide a safety valve in a shut-off valve in order to prevent excessive pressure when the shut-off valve is closed suddenly.

It is a still further object of the present invention to provide a safety valve which operates independently of the speed of rotation of the fluid engine which it is protecting.

With these objects in view a hydraulic drive, according to the present invention, mainly comprises a fluid pump and a fluid engine, each having an intake port and a discharge port, and being operated by a working fluid, supply conduit means connecting the discharge port of the fluid pump with the intake port of the fluid engine for transporting the working fluid from the fluid pump to the fluid engine, return conduit means connecting the discharge port of the fluid engine with the intake port of the fluid pump for transporting the working fluid from the fluid engine to the fluid pump, shut-oh" valve means located in the return conduit means adjacent to the discharge port of the fluid engine and being movable between two positions, namely, an open position permitting free passage of the working fluid through the return conduit means and a closing position closing the return conduit means, reservoir means for containing a predetermined amount of the working fluid, connecting conduit means connecting the reservoir means with a point of the return conduit means located betw en the shut-off valve means and. the. in ake 1 2 port of the fluid pump permitting flow of the working fluid from the return conduit into the reservoir means and back, and control means operatively connected to the shut-off valve means for operating the same and actuated by the variations of the fluid conditionsv in the reservoir means so as to move, the shut-off valve means into closing position when fluid flows out of the reservoir means through the conduit means into the return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

According to a preferred embodiment of the present invention the safety device is part of a hydraulic drive including a fluid pump and a fluid engine, each having an intake port and a discharge port, and being operated by a working fluid, supply conduitv means connecting the dis,- charge port of the fluid pump with the intake port of the fluid engine for transporting the working fluid from the fluid pump to the fluid engine, and return conduit means connecting the discharge port of the fluid engine with the intake port of the fluid pump for transporting the working fluid from the fluid engine to the fluid pump, and comprises in combination, shut-off valve means located in the return conduit means adjacent to the discharge port of the fluid engine and being movable between two positions, namely, an open position permitting free passage of the working fluid through the return conduit means and a closing position closing the return conduit means, reservoir means for containing a predetermined amount v of the working fluid, connecting conduit means connecting the reservoir means with a point of the return conduit means located between the shut-off valve means and the intake port of the fluid pump permit.- ting flow of the working fluid from the return conduit into the reservoir means and back, and control means operatively connected to the shutoff valve means for operating the same and actuated by the variations of the fluid condutions in the reservoir means so as to move the shut-01f valve means into closing position when fluid flows out of the reservoir means through the conduit means into the return conduit means in the event that part of the working fluid circulatin in the hydraulic drive is lost. I

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as tov its construction and its method of operation, together with additional obje s and advantages thereof, will. be

a best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. l is a diagrammatic sectional view through a hydraulic drive provided with a safety device according to a preferred embodiment of the present invention;

Fig. 2 is a partial cross-sectional view showing a shut-oif valve on a large scale;

Fig. 3 is a cross-sectional view through the fluid engine of the hydraulic drive taken on line 33 of Fig. 4;

Fig. 4 is a longitudinal sectional view through the fluid engine of the hydraulic drive; and

Fig. is a cross-sectional view taken on the line 55 of Fig. 4.

Referring now to the drawings, Fig. 1 shows a fluid reservoir I in which a float 2 is arranged for rotation about a pivoting point 3 by means of a lever 2. One end of the lever 2' is projecting out of the reservoir and operates a contact 4 according to the movements of the float 2. When the contact ii is closed the circuit of the battery 6' is closed and the electro-magnet fiis operated. A connecting conduit 27 connects the reservoir 2 with a point of the return conduit 9 adjacent to the fluid pump 25. A supply conduit 26 connects the fluid pump 25 with the fluid engine 5. The discharge port of the fluid engine 5 is connected to the return conduit 9, which is provided with a recessed portion 9' adjacent to the discharge port of the fluid engine. A shutoff valve 8 is pivotally mounted in the recess 9 movable between an open position located in recess 9' and permittin free flow in the return conduit 9, and a closed position transversal to the axis of the return conduit and closing the same. A pivotally mounted lever l, secured to the valve 6, is actuated by a spring It to move the shut-off valve 8 in clockwise direction and to hold it in open position. The electro-magnet 6 is operatively connected to the lever l by means of a rod 5" in such manner that when contact 4 is closed, lever l is turned counterclockwise against the action of spring it) by rod 5", whereby the shut-off valve 8 is turned counterclockwise and closed.

In the event that a leakage occurs in the hydraulic drive, fluid contained in the reservoir I flows through conduit 2'! into the return conduit 9 and the level in the reservoir I drops. As soon as the level in the reservoir drops below the predetermined level of the float 2 the contact l is closed by means of lever 2', and electro-magnet 6 closes the shut-off valve 8.

In the event that the return conduit 9 breaks, which may occur when the fluid engine is braked, the level in reservoir 1! sinks rapidly and the shutoff valve 8 is actuated immediately whereby the fluid engine is stopped and an accident caused by sudden dropping of a load carried by a hoisting apparatus driven by the fluid engine may be avoided. H

Reservoir I also performs a function as a balancing container and receives through conduit 2'! any excess amount of fluid developing when the fluid circulating in the hydraulic drive expands owin to a raise of temperature.

If the quantity of fluid delivered by the fluid engine is very large, intolerably high pressure may develop when the shut-off valve is suddenly closed owing to a break in the conduit system.

A small passage could be provided in the shutoff valve permitting a small amount of fluid to pass through in closed'position of the shut-01f valve in order to prevent excess pressure. This arrangement would have the disadvantage that the braking power of the fluid engine would then be dependent on the speed of rotation of the fluid engine at the moment in which the shut-off valve was closed.

According to a preferred embodiment of the present invention a spring loaded safety valve is provided in the shut-off valve in order to prevent excess pressure when the shut-off valve is suddenly closed whereby the above-described disadvantage of an uncontrolled passage through the shut-off valve is overcome.

The safety valve is preferably constructed as a ball valve H as shown in Figs. 1 and 2. After closure of the shut-off valve 8 the ball [2, which is pressed by spring l2 towards the valve seat 28, is moved by the fluid pressure in the fluid engine against the action of spring 12 away from valve seat 28 permitting flow of fluid through valve seat 28 and the bore it in the member 13 into the return conduit. The construction of the safety valve is such that the velocity of the fluid passing through the openings 28 and I 3' is low, and that the pressure of the spring I2 increases only little when it is depressed by the movement of the ball l 2. This results in a braking action which is almost independent of the speed of rotation of the fluid engine.

Figs. 3 and 4 show a fluid engine which may be used for driving a cable drum or a hoisting apparatus by means of a pulley IS. The fluid enters the fluid engine through intake port M and flows through slots it provided in the inner wheel !3 into the chambers 28 between inner wheel l8 and outer wheel [1. Chambers 20 diminish in size on rotation of the wheels ii and I8, and press the fluid contained therein through slots 2| into the discharge port 22. Adjacent thereto the shut-off valve 3 is arranged. The fluid flows through conduit 9 to the fluid pump, or through connecting conduit 21 to the reservoir as long as shut-off valve 8 is open.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of safety devices for a hydraulic drive diifering from the type described above.

'While the invention has been illustrated and described as embodied in a shut-off valve provided with a safety valve and arranged in the return conduit of a hydraulic drive, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applyin current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

conduit means connecting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump to said fluid engine; return conduit means connecting said discharge port of said fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid pump; shutofi valve means located in said return conduit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of said return conduit means located between said shut-off valve means and said intake port of said fluid pump permitting flow of said working fluid from said return conduit into said reservoir means and back; and control means operatively connected to said shut-off valve means for operating the same and actuated by the variations of the fluid conditions in said reservoir means so as to move said shut-off valve means into closing position when fluid flows out of said reservoir means through said connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

2. A hydraulic drive, comprising in combination, a fluid pump and a fluid engine, each having an intake port and, a discharge port, said fluid engine being operated by a working fluid; supply conduit means connecting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump, to said fluid engine; return conduit means connecting said discharge port of said fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid pump; shut-oil valve means located in said return con duit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting tree passage of said working fluid through said return conduit means and a closing position closing said return conduit means; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a pointof said return conduit means located between said shut-ofl valve means and said intake port of said fluid pump permitting flow of said working fluid from said return conduit into ,said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein; and control means operatively connecting said float means and said shut-ofl valve means and actuated by the movement of said float means so as to move said shutoff valve means into closing position when the level of the fluid contained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

3. A hydraulic drive, comprising in combination, a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a working fl id; supplyconduit meansconneoting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump to said fluid engine; return conduit means connecting said discharge port of said fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid pump; shut-off valve means located in said return conduit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of said return conduit means located between said shutofl valve means and said intake port of said fluid pump permitting flow of said working fluid from said return conduit into said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein; electro-magnet means operatively connected to said shut-ofi valve means; and electric contact means arranged in the circuit of said electromagnet means and actuated by the movement of said float means for energizing said electro-magnet means so as to move said shut-off valve means into closing position when the level of the fluid contained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

4. A hydraulic drive, comprising in combination, a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a working fluid; supply conduit means connecting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump to said fluid engine; return conduit means connecting said discharge port of said fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid-pump; shut-off valve means located in said return conduit means adjacent to the discharge port of said fluid engine and being movable between, two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means; means urging said shut-ofl valve means into said open position; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with .a point of said return conduit means located between said shut-off valve means and said intake port of said fluid pump permitting flow of said working fluid from said return conduit into said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein; and control means operatively connecting said float means and said shut-off valve means and actuated by the movement of said float means so as to move said shut-off valve means into closing position when the level of the fluid contained in said reservoir means drop as a result of working fluid flowing out of said reservoir means through said connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

5. A hydraulic drive, comprising in combination, a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a working fluid; supply conduit means connecting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump to said fluid engine; return conduit means connecting said discharge port of said fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid pump; shut-ofl valve means located in said return conduit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means;

spring means urging said shut-off valve means into said open position; reservoir means for containing a predetermined amount of said working fluid; connecting means connecting said reservoir means with a point of said return conduit means located between said shut-off valve means and said intake port of said fluid pump mermitting flow of said working fluid from said return conduit into said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein; electro-magnet means operatively connected to said shut-off valve means; and electric contact means arranged in the circuit of said electro-magnet means and actuated by the movement of said float means for energizing said electro-magnet means so as to move said shut-off valve means into closing position against the action of said spring means when the level of the fluid con tained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

6. A hydraulic drive, comprising in combination, a fluid pump and a fluid engine, each hav: ing an intake port and a discharge port, said fluid engine being operated by a working fluid;

turn conduit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means, said shut-ofl valve means being provided with a passage; a spring loaded safety valve located in said passage in said shut-off valve means and closing the same, but permitting said passage to open when the fluid pressure of said hydraulic engine exceeds a predetermined maximum pressure; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of said return conduit means located between'said shut-off valve means and said intake port of said fluid pump permitting flow of said working fluid from said return conduit into said reservoir means and back; and control means operatively connected to said shut-off valve means for operating the same and actuated by the variations of the fluid conditions in said reservoir means so as to move said shut-01f valve means into closing position when fluid flows out of said reservoir means through said connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

7. A hydraulic drive, comprising in combination, a fluidpump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a working fluid; supply conduit means connecting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump to said fluid engine; return conduit means connecting said discharge port of said fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid pump; shutofl valve means located in said return conduit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means, said shut-off valve means being provided with a passage; 9, safety valve located in said passage in said shut-off valve means and including a ball-shaped closure member movable between a closing position seated on a valve seat in said passage and an open position permitting working fluid to pass through said passage in said shut-off valve, and a spring urging said closure member into said closing position but being adjusted so as to permit said working fluid to move said ball-shaped closure member into said open position when the fluid pressure of said hydraulic engine exceeds a predetermined maximum pressure; spring means urging said shut-ofl valve means into said open position; reservoir means for containing a, predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of said return conduit means located between said shut-off valve means and said intake port of said fluid pump permitting flow of said working fluid from said return conduit into said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein; electromagnet means operatively connected to said shutofi valve means; and electric contact means arranged in the circuit of said electro-magnet means and actuated by the movement of said float means for energizing said electro-magnet means so as to move said shut-off valve means into closing position against the action of said spring means when the level of the fluid contained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said conduit connecting means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

8. A hydraulic drive, comprising in combination, a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a working fluid; supply conduit means connecting said discharge port of said fluid pump with said intake port of said fluid engine for transporting said working fluid from said fluid pump to said fluid engine; return conduit means connecting said discharge port of said-fluid engine with said intake port of said fluid pump for transporting said working fluid from said fluid engine to said fluid pump; shutofi' valve means located in said return conduit means adjacent to the discharge port of said fluid engine and being movable between two positions, namely, an open position permitting free passage of said working fluid through said return conduit means and a closing position closing said return conduit means, said shut-off valve means being provided with a passage; a safety valve located in said passage in said shut-off valve means and including a ball-shaped closure member movable between a c osing position seated on a valve seat in said passage and an open position permitting working fluid to pass through said passage in said shut-01f valve, and a spring urging said closure member into said closing position but being adjusted so as to permit said working'fluid to move said ball-shaped closure member into said open position when the fluid pressure of said hydraulic engine exceeds a predetermined maximum pressure; spring means urging said shut-oif valve means into said open position; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of said return conduit means located between said shut-off valve means and said intake port of said fluid pump permitting flow of said working fluid from. said return conduit into said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein, and including a lever pivotally mounted in the wall of said reservoir means and projecting out of said reservoir means; electro-magnet means operatively connected to said shut-oil valve means; and electric contact means arranged in the circuit of said electro-magnet means and actuated by the movement of said lever of said float means for energizing said electro-magnet means so as to move said shut-oi? valve means into closing position against the action of said spring means when the level of the fluid contained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said. connecting conduit means into said return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

9. In a hydraulic drive including a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a working fluid, supply conduit means connecting the discharge port of the fluid pump with the intake port of the fluid engine for transporting the working fluid from the fluid pump to the fluid engine, and return conduit means connecting the discharge port of the fluid engine with the intake port of the fluid pump for transporting working fluid from the fluid engine to the fluid pump, in combination, a safety device comprising shut-off valve means located in the return condui means adjacent to the discharge port of the fluid engine and being movable between two positions, namely, an open position permitting free passage of the working fluid through the return conduit means and a closing position closing the return conduit means; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of the return conduit means located between said shut-off valve means and the intake port of the fluid pump permitting flow of the Working fluid from the return conduit into said reservoir means and back; and control means operatively connected to said shutoff valve means for operating the same and actuated by the variations of the fluid conditions in said reservoir means so as to move said shut-oi? valve means into closing position when fluid flows out of said reservoir means through said connecting conduit means into the return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

10. In a hydraulic drive including a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a Working fluid, supply conduit means connecting the discharge port of the fluid pump with the intake port of the fluid engine for transporting the working fluid from the fluid pump to the fluid engine, and return conduit means connecting the discharge port of the fluid engine with the intake port of the fluidpump for transporting the working fluid from the fluid engine to the fluid pump, in combination, a safety device comprising shut-oif valve means located in the return conduit means adjacent to the discharge port of the fluid engine and being movable between two positions, namely, an open position permitting free passage of the working fluid through the return conduit means and a closing position closing the return conduit means; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of the return conduit means located between said shut-otf valve means and the intake port of the fluid pump permitting flow of the working fluid from the return conduit into said reservoir means and back; float means arranged in said reservoir means for movement according to variations of the level of the working fluid contained therein; electro-magnet means operatively connected to said shut-01'1" valve means; and electric contact means arranged in the circuit of said electro-magnet means and actuated by the movement of said float means for energizing said electrc-magnet means so as to move said shutoff valve means into closing position when the level of the fluid contained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said conduit means into the return connecting conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

11. In a hydraulic drive including a fluid pump and a fluid engine, each having an intake port and a discharge port, said fluid engine being operated by a Working fluid, supply conduit means connecting the discharge port of the fluid pump with the intake port of the fluid engine for trans-; porting the working fluid from the fluid pump to the fluid engine, and return conduit means connecting the discharge port of the fluid engine with the intake port of the fluid pump for transporting the working fluid from the fluid engine to the fluid pump, in combination, a safety device comprising shut-ofl valve means located in the return conduit means adjacent to the discharge port of the fluid engine and being movable between two positions, namely, an open position permitting free passage of the working fluid through the return conduit means and a closing position closing the return conduit means, said shut-ofi valve means being provided with a passage; a safety valve located in said passage in said shut-off valve means and including a ball-shaped closure member movable between a closing position seated on a valve seat in said passage and an open position permitting working fluid to pass through said passage in said shutoff valve, and a spring urging said closure member into said closing position but being adjusted so as to permit the working fluid to move said ball-shaped closure member into said open position hen the fluid pressure of the hydraulic engine exceeds a predetermined maximum pressure; spring means urging said shut-off valve means into said open position; reservoir means for containing a predetermined amount of said working fluid; connecting conduit means connecting said reservoir means with a point of the return conduit means located between said shut-0E valve means and the intake port of the fluid pump permitting flow of the working fluid from the return conduit into said reservoir means and back; float means arranged in said reservoir means for movement accordin to variations of the level of the working fluid contained therein, and including a lever pivotally mounted in the wall of said reservoir means and projecting out of said reservoir means; electro-magnet means operatively connected to said shut-off valve means; a d electric contact means arranged in the circuit of said electro-magnet means and actuated by the movement of said lever of said float means for energizing said electro-magnet means so as to move said shut-off valve means into closing position against the action of said spring means when the level of the fluid contained in said reservoir means drops as a result of working fluid flowing out of said reservoir means through said connecting conduit means into the return conduit means in the event that part of the working fluid circulating in the hydraulic drive is lost.

THEODOR KLA'ITE.

o'r'ro NiiBLING.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 925,053 Sturgis June 15, 1909 1,284,197 Larner et a1. Nov. 5, 1918 1,787,565 Brown Jan. 6, 1931 

